The fatigue life of a work piece having a hole therethrough can be increased by generating compressive stress gradients within the work piece about the hole. This has the effect of increasing the ductility of the material about the hole, and since the greatest stress concentration due to external loading occurs at the hole surface, this increased ductility serves to increase the fatigue life of the work piece.
One technique that has been used to generate these compressive stress gradients is to force a fastener into the hole where the shank of the fastener has a diameter larger than the initial diameter of the hole to produce what is commonly known as an interference fit. The fastener thus exerts a radial outward force on the material about the hole to set up a radially directed compressive stress gradient in the material of the work piece about the hole to increase the fatigue life thereof. Two problems are encountered in using interference fit fasteners. The first is that, while the fastener radially and circumferentially expands the hole to generate the desirable compressive stress gradient, it also axially elongates the grains of the material at the hole surface to generate an undesirable axially oriented tensile stress gradient in these surface grains. Because tensile stress gradients in a work piece tends to decrease its ductility and thus its fatigue life, the advantageous radially oriented compressive stress gradient that is introduced by a fastener in interference fit as partially offset by the axially oriented tensile stress gradient introduced as the fastener is pressed into the hole. The second problem is that because external loading of the work piece causes a small amount of movement between the fastener shank/work piece interface, the material of the work piece at the interface will be subjected to a phenomenom called fretting, significantly decreasing the fatigue life of the work piece.
Another technique that has been used to generate compressive stress gradients in the material of the work piece about the hole is to pass an expansion mandrel having an expansion section thereon with a diameter larger than the hole axially through the hole so that the material immediately around the hole is elongated both axially and circumferentially beyond its elastic limit to cause the material immediately adjacent the hole to be permanently deformed in its expanded condition. After the mandrel has passed through the hole, the material of the work piece farther away from the hole that is not stressed beyond its yield point elastically springs back attempting to close the hole. This supplied a compressive radial load on the material immediately adjacent the hole to place a compressive stress gradient in at least part of that material immediately adjacent the hole. Such a technique is called coldworking. While the radially directed compressive stress gradient increases the fatigue life of the material about the hole, the axial elongation of the surface grains of the material immediately adjacent the hole retain their axially oriented tensile stress gradient. Thus, like the interference fit fastener, the desirable radially oriented compressive stress gradient is at least partially offset by the undesirable axially oriented tensile force gradient.
Another technique that has been used to generate the compressive stress gradient in the material adjacent the surface of the hole is a modified coldworking technique which uses a pre-split tubular member, either axially split or helically split. The tubular member is placed through the hole in the work piece prior to expansion and then the coldworking mandrel with the expansion section thereon larger than the inside diameter of the tubular member is passed through the inside of the tubular member to expand the tubular member outwardly into contact with the material of the work piece at the hole surface and then expand the material of the work pieces at the hole surface sufficiently to permanently elongate the grains of the material of the work piece at the hole surface. After the mandrel has passed through the pre-split tubular member, the tubular member is removed. Two problems are encountered when using this technique. The first is that the interruption in the tubular member surface at the split produces a reduced compressive stress gradient along the material at the surface of the hole and the second is that because the tubular member is allowed to move slightly axially within the hole during the coldworking operation, the axially oriented tensile stress gradient is still induced into the surface grain of the material about the hole. Thus, the desirable compressive stress gradient induced into the material at the surface of the hole is partially offset first by the lower compressive stress gradient occurring at the split in the sleeve and secondly by the axially oriented tensile stress gradient induced into the grain of the material at the surface of the hole.
Various methods and techniques are available on the market today for accurately sizing holes in work pieces for the installation of fasteners therein. The most commonly used technique is to drill a pilot hole through the work pieces and then ream this pilot hole to final size with a reamer. While drilling and reaming can be satisfactorily used in a laboratory or machine shop environment where sufficient equipment is available to both accurately drill and ream the holes, this technique has been unsatisfactory when the operation must be performed using manually held tools even though elaborate guides have been devised. This has been especially true in the aerospace industry where the use of compound surfaces and the necessity of field repair have required widespread use of manually held tools. As a result of the use of such manually held tools, widespread use of nonstandard sizes of fasteners have been required to correct improperly sized holes.
Another hole sizing technique that has been attempted is to drill a pilot hole and then pull a mandrel with an expansion section thereon through the pilot hole to finally size same. Because the material of the work pieces rebounds after the passage of the mandrel through the holes and the amount of rebound is a function of the amount the pilot hole is enlarged, it is necessary to accurately size the holes either before or after the enlarging operation with a tool such as a reamer in order to accurately determine the final hole size. Thus, because of the inability to accurately control the reaming operation when using manually held tools, this technique has been unable to accurately size holes.